This invention relates to the control of nuclear reactors and more particularly to the closed-loop control of reactor power.
To change the steady state power level of a nuclear reactor, neutron-absorbing control elements such as control rods or shim blades are moved in and out of the rector core. Also, certain soluble chemicals may be used as the control element. Also, rotating drums may be used as the control elements. For example, to increase the power of a nuclear reactor, the control rods are moved outwardly so that the neutron population can build up to a level consistent with the higher output level. Once the population has increased to the suitable level, the control rods are reinserted to their original position less the impact of feedback effects such as temperature and voids on the reactor's neutron population. Alternatively, if chemical poisons were being used, the concentration of the chemical would be decreased, the power allowed to rise, and the concentration then restored to its initial value less feedback effects. Alternatively, if rotating drums were being used, the drums would be rotated so that their neutron-absorbing sides were positioned away from the reactor and their neutron-reflecting sides positioned towards the reactor and the power allowed to rise. Once the desired power level has been attained, the drums are returned to their original positions less feedback effects. The reactor will then continue to operate at the new higher output level. Normally, because of the nuclear reactor dynamics to be discussed in more detailed below, the direction of control rod movement or the change in concentration of the chemical, such as boric acid, or the rotational movement of the drums must be reversed before the new power level has been attained. Moreover, this reversal is usually not accomplished as one continuous movement but in a series of short reversals interrupted by intervals in which the rod position or chemical concentration or drum orientation is kept constant. Once the reversal in the direction of rod travel or the change in chemical concentration or the change in drum orientation has been initiated, the power may continue to build up but at a slower rate. If the changes are performed properly, the power level will climb to the desired new operating level without overshoot or undershoot. At the present time decisions concerning control mechanism reversal are made by the licensed reactor operator based on knowledge of the dynamics of the reactor and the experience of the operator. These decisions are complicated because (1) the equations of reactor dynamics are non-linear, (2) the rate of change of power depends on both the net change in the control mechanism's position, concentration, or orientation and on its rate of change, (3) there are feedback effects between the reactor power and the rate at which power is changing, and (4) control mechanisms have non-linear strengths and finite speeds.
Recently, a methodology has been demonstrated (U.S. Ser. No. 633,939 filed July 24, 1984 and U.S. Ser. No. 907,048 filed Sept. 12, 1986) that provides a closed-loop control system for regulating reactor power which results in the attainment of new power levels without overshoot. That methodology which is referred to as the `Reactivity Constraint Approach` is a two-part process involving a supervisory controller and an associated control law. The invention disclosed herein pertains to the development and demonstration of a closed-loop control law that is compatible with the `Reactivity Constraint Approach` and which permits the reactor power to be adjusted in minimum time.
It is therefore an object of this invention to provide a closed-loop control law for regulating reactor power in a nuclear reactor.
It is a further object of the invention to provide such a closed-loop law which results in the attainment of new power levels without overshoot (or conversely, undershoot) beyond that allowed by specified tolerance bands.
It is yet another object of the invention to provide such a closed-loop control law which when applied to reactors that are subject to a limitation on the maximum allowed rate of change of power results in the attainment of new power levels in minimum time.
Still another object of the invention is a closed-loop control law that is compatible with the `Reactivity Constraint Approach` as described by U.S. Ser. Nos. 633,939 and 907,048.
Yet another object of the invention is a closed-loop control law which recognizes that reactor dynamics are non-linear, that the rate of change of reactor power depends both on the net reactivity and the rate of change reactivity, that the reactivity is dependent on the reactor power through various feedback mechanisms, and that control mechanisms have finite speeds as well as position-dependent, non-linear worths.